Coordinated Responses of The CVS * Flashcards
What is orthostasis?
Standing up
What are the effects of orthostasis on the blood flow to the brain?
- CVS experiences changes due to gravity
- Postural hypotension causing lack of blood flow to the brain due to fall in blood pressure
- In extreme cases, fainting
How does the body counteract postural hypotension caused by orthostasis?
- Fall in BP recovers due to homeostatic mechanisms such as baroreflex
- Increases heart rate, force of contraction and TPR
Describe the effects of orthostasis on blood pressure on different parts of the body.
- Blood pressure is lowest at the head and highest at the feet.
- REASON: force of gravity pulling blood down towards the feet
- More blood pooling at the feet
- Applies a greater hydrostatic pressure on the vascular wall
How is Bernoulli’s law used to counteract the pooling of blood at the feet during orthostasis?
- Increase in potential energy (from heart to feet)
- Increase in kinetic energy of ejected blood
- Increases blood flow
Why can orthostasis sometimes cause fainting? PART 1
- Blood starts to pool in the legs under the force of gravity
- CVP decreases
- Less blood returns to the heart so EDV decreases
- Less filling so less stretch in the heart
Why can orthostasis sometimes cause fainting? PART 2
- Force of contraction would be weaker (Starling’s law)
- Decrease in stroke volume
- Decrease in cardiac output
- Poor perfusion of the brain causing fainting
How does lying down/fainting counteract hypotension? PART 1
- Blood is evenly distributed in the veins
- CVP increases
- Greater filling
- EDV increases
- Greater stretching of vascular muscles.
How does lying down/fainting counteract hypotension? PART 2
- Increased force of contraction
- Increased stroke volume
-Increased cardiac output - Better perfusion of the brain
How do alpha-adrenergic blockers make postural hypotension worse?
- Reduce vascular tone
- Inhibit the body’s ability to respond to an increase in vascular tone
How can impairment of varicose veins make postural hypotension worse?
Impairs venous return as more blood will pool in the veins
How can a lack of skeletal muscle activity make postural hypotension worse?
- Occurs due to paralysis or forced inactivity (e.g. long term bed rest)
- Muscles required to help pump blood into the heart
- Inactivity reduces the amount of blood leading to the heart
How can a reduction in circulating volume make postural hypotension worse?
Reduces preload and so baroreceptors are not able to respond to changes (e.g. haemorrhage)
How can increased core body temperatures make postural hypotension worse?
- Peripheral vasodilation
- Reduces the amount of blood going to the heart
- Less stroke volume and cardiac output
What are the initial effects of microgravity (space) on the CVS? PART 1
- Blood returns to the heart
- Increases atrial and ventricular volume
- Increased cardiac output.
What are the initial effects of microgravity (space) on the CVS? PART 2
- Sensed by cardiac mechanoreceptors which reduce sympathetic activity.
- Reduces ADH and increases ANP which increases GFR and reduces RAAS.
- Overall reduction in blood volume
What are the long term effects of microgravity on the CVS?
- Lower blood volume as there is reduced stress on the heart.
- Causes the heart to reduce in muscle mass
- Causes BP to drop.
Some astronauts suffer from postural hypotension after returning to Earth from space. Suggest why.
- Smaller blood volume and smaller heart due to long-term effects of being in space (microgravity).
- Baroreceptors cannot compensate for this
What does the body aim to do when exercising?
Mechanisms occur that increase blood supply to exercising muscle
What causes the changes in cardiovascular activity in the body during exercise?
- Central command in the brain in response to stimuli (e.g. anticipation of exercise)
- Feedback from muscles through mechanoreceptors and metaboreceptors
- This affects sympathetic activity and vagus inhibition
What changes do we see in the CVS during exercise? PART 1
Increase lung oxygen uptake
- Oxygen is transported around the body to supply exercising muscles.
- Requires an increased HR and increased force of contraction
What changes do we see in the CVS during exercise? PART 2
Constant control of BP
By controlling BP, can increase cardiac output and protect the heart from excessive damage caused by increased BP (arising from increased HR)
What changes do we see in the CVS during exercise? PART 3
Co-ordinated dilation/constriction of vascular beds
- Allows selective targeting of areas to which oxygen is delivered i.e muscles
How is oxygen uptake into the lungs increased during exercise? PART 1
- Increase in heart rate and stroke volume
- Blood pumped around the body at a faster rate
- Oxygen gets used up by respiring tissue more quickly
How is oxygen uptake into the lungs increased during exercise? PART 2
- Increase difference in arteriovenous oxygen difference.
- The bigger the difference in oxygen concentration in the arteries and veins the bigger the concentration gradient
- Faster diffusion.
How does arterio-venous oxygen difference change with increasing intensity of exercise? (REFER TO CURVE ON SLIDES)
- During light exercise the arterio-venous oxygen difference is low
- From light to moderate intensity this oxygen difference increases very steeply
- From moderate to heavy the curve is now less steep but still continues to rise
- At heavy exercise the curve starts to plateau
How does cardiac output change with increasing intensities of exercise?
- During light exercise - very low cardiac output.
- From light to medium intensity - small increase in cardiac output
- From medium to heavy - greater increase in cardiac output
- Heavy exercise - cardiac output increases dramatically
How does stroke volume change with increasing intensities of exercise? PART 1
- During light exercise - very low stroke volume
- From light to medium intensity - large increase in stroke volume due to Starling’s law (increased stretching)
How does stroke volume change with increasing intensities of exercise? PART 2
- From medium to heavy - smaller increase in stroke volume as there is a decreased cardiac output due to reaching elastic limit of Starling’s law
- Heavy exercise the stroke volume starts to decrease
How does heart rate change with increasing intensities of exercise?
- During light exercise - very low HR
- From light to medium intensity - small increase in HR
- From medium to heavy - greater increase in HR
- Heavy exercise - HR increases dramatically
What is fast heart rate called?
Tachycardia
How is tachycardia achieved?
- Caused by the brain central command
- Decrease in the signal down the vagus nerve to the SA and AV nodes
- Increase in sympathetic activity to the SA and AV nodes
- Causes the heart rate to increase
How is stroke volume increased using EDV?
- High EDV so filling pressure is high
- Increased sympathetic activity and other systems e.g the calf muscle pump causes venoconstriction
- Increases CVP
- Starling’s law increases preload
How does faster ejection lead to a greater stroke volume?
- Increased sympathetic activation of β-1 receptors
- Inotropic Ca2+ to increase the speed of ejection
How is stroke volume increased using ESV?
- Decreased end-systolic volume so increased ejection
- Increased contractility by sympathetic activation of β-1 receptors
- Increases stretching
What are the effects of an increase in cardiac output during exercise?
- Overall decrease in TPR
- Blood pressure doesn’t increase by much
- Systolic and diastolic volumes increase
- Stroke volume and filling are greater
What is compensatory vasoconstriction?
- The vasoconstriction/blocking of blood supply to less active parts of the body during exercise i.e
- Prevents hypotension as exercise causes decreased TPR so prevents the BP from getting too high/low
What are metaboreceptors?
Small chemosensitive sensory fibres in skeletal muscle
What are metaboreceptors stimulated by
Stimulated by K+, H+ and lactate (which increase in exercising muscle)
What reflex actions do metaboreceptors cause?
- Tachycardia due to increased sympathetic activity
- Increased blood pressure
- Pressor response to exercise
What is the reflex response to orthostasis? PART 1
- Less stimulation (unloading of baroreceptors)
- Lower afferent fibre activity
- Signal goes to NTS
- Switches off inhibitory nerves that go from CVLM to the RVLM.
- Results in RVLM being more active sending efferent signals to heart and arterioles.
What is the reflex response to orthostasis? PART 2
- Increased sympathetic drive to SA node and increased HR.
- Myocardium increased contractility
- Vasoconstriction (arterioles, veins) increases TPR.
- Less vagal parasympathetic activity to SA node – overall increase in blood pressure.
What is dynamic exercise associated with?
Constantly shortening and of different muscle groups– associated with a lower BP, lower sympathetic tone
- Involves ‘lower loaded’ exercise
What is static exercise associated with?
One specific muscle group is being worked without constant movement – associated with higher BP and metabolic hyperaemia
Describe the two different ways vasodilation occurs.
- Fall in local resistance due to metabolic hyperaemia
- Local sympathetic response can also occur - β2-mediated vasodilation via circulating adrenaline.
- β2 receptor expression high in skeletal muscle and coronary artery.
When are metaboreceptors particularly important?
Isometric exercise
hat is the effect of an increase in BP caused by metaboreceptors?
- Raised BP maintains blood flow to contracted muscle
- Contracted muscle also supplied by dilated resistance vessels due to metabolism - selective metabolic hyperaemia.
